Some examples of nonvolatile memory devices using resistance materials include resistive random access memory (RRAM) devices, phase-changeable random access memory (PRAM) devices and magnetic random access memory (MRAM) devices. Dynamic random access memory (DRAM) devices or flash memory devices store data by using charges, while nonvolatile memory devices use resistance materials to store data using a change in resistance of a variable resistance material (RRAM), a state change of a phase change material like a chalcogenide alloy (PRAM) or a change in resistance of a magnetic tunnel junction (MTJ) thin film according to the magnetization of a ferromagnetic material (MRAM), for example.
Here, resistive memory cells include variable resistance materials between an upper electrode and a lower electrode, and a resistance level of the variable resistance materials changes according to a voltage that is applied to the upper and lower electrodes. Examples of such resistive memory cells are disclosed in U.S. Patent Publication No. 2005/0058009 and U.S. Patent Publication No. 2004/0027849. In particular, a filament that serves as a current path of a cell current is formed in the variable resistance material. A state in which a part of the filament is disconnected may be defined as a reset state, a high-resistance state, and reset data (data 1). A state in which the filament is connected may be defined as a set state, a low-resistance state, and set data (data 0).
A reset voltage that has a voltage level at which the filament may be disconnected is supplied to write reset data into the resistive memory cells. A set voltage that has a voltage level at which the filament may be reconnected is supplied to write set data into the resistive memory cells. Further, a voltage that has a voltage level that is too low to change the state of the filament is supplied to read the stored data to find out whether the read data is reset data or set data.